This invention relates generally to amplitude-modulated (AM) signal receivers having high sensitivity and selectivity and providing high-fidelity reproduction.
As is well known, in order to increase the sensitivity and selectivity of a radio receiver the value of Q in the input tuning circuit should be made as high as possible. However, a very sharp selectivity suffers from a drawback that high-fidelity (hi-fi) in the demodulated audio signals is apt to be lost. Since the selectivity of AM signal receivers is usually set to a relatively high value so that practical receiving ability is ensured even if the receiver is used at a point of low radio field strength, hi-fi reproduction could not be obtained even if the receiver is used at a point of high radio field strength. Although it is theoretically possible to simply lower the Q value of the input tuning circuit when it is intended to widen the tuned frequency range to obtain higher-fidelity when the radio field strength is sufficiently high so that extremely sharp selectivity and sensitivity are unnecessary, such a simple decrease in Q will result in deterioration in tuning operation. Namely, assuming that the listener wishes to tune the receiver on a broadcasting signal frequency whose intensity is relatively low while the objective signal frequency is close to another broadcasting signal frequency having relatively high intensity, it would be impossible to select the objective signal if the value of Q of the input tuning circuit were lowered.
Apart from the above problem, when it is intended to receive an amplitude-modulated stereo signal with an AM stereo receiver, it is desirable if the hi-fi characteristic of the receiver is improved as long as the selectivity and sensitivity are maintained above necessary values.